Thread: Help with a linked list.

Hello. I'm having trouble with a simple linked list. I have two functions. One creates and populates the list. The other prints the list. For trouble shooting, I have commented out the function that prints the list and instead I've tried to print it inside the function that creates and populates it. Here's the code:

Code:

#include <stdio.h>
#include <stdlib.h>
#include <stdint.h>

struct IRD {
    uint8_t num_region_hashes;
} ird;

typedef struct REGION {
    int val;
    struct REGION *next;
} regions;

regions *head = NULL;
regions *ptr = NULL;

void create_list(regions *head, uint8_t num_region_hashes);
void print_list(regions *head);

int main() {
    ird.num_region_hashes = 4;

    create_list(head,ird.num_region_hashes);
/*
 *   print_list(&temp);
 */

}

void create_list(regions *head, uint8_t num_region_hashes) {
    uint8_t a = 1;

    head = malloc(sizeof(regions ));

    while(a!=num_region_hashes) {
        ptr = malloc(sizeof(regions ));
        ptr->val = a;
        printf("ptr->val = %d.\n",ptr->val);
        if(a==1) {
            head = ptr;
        }
        ptr->next = ptr;
        a++;
    }

    ptr->next = NULL;

    while(head->next != NULL) {
        printf("head->val = %d.\n",head->val);
        head = head->next;
    }
    printf("Leaving create_list.\n");
}

void print_list(regions *head) {

    while (head->next != NULL) {
        printf("%d\n", head->val);
        head = head->next;
    }

    printf("Leaving print_list.\n");
}

In the create_list function, when I try printing the linked list, all it does is display 1 and loop for some reason. Any ideas what I'm doing wrong? Thanks.

Essentially the code in the create_list function is a bit of a mess. You have left head pointing to itself, so your loop to print the list just keeps printing out the head value in a continuous cycle. You really need to sit down with paper and pen and go through the logic you need step by step.

Thank you for the reply. I will do that. I lost all of my programming books in a flood a while back. I've read a bit on the internet about linked lists. I think I need to find a good book (whether it's a hard copy or a digital copy) and maybe study them a bit more in depth. Thanks again for the help.

A lot of programmers, including myself, find it helps to draw out the linked list on paper first with all the connections that need to be made and then use that as a plan to work out the necessary code.